The present invention relates to a coating composition that is fast curing at ambient temperature (touch dry in less than 2 hours at 25° C.) with a high solids content (>70% by weight) and a low VOC (<250 grams solvent per litre of the composition, g/l) which can be used in durable protective coatings.
There has been increased concern in recent years about the release of volatile solvents into the atmosphere, and there has been a consequent need to reduce the volatile organic solvent content of coating, sealant, and adhesive compositions. This has not been easy for coating compositions, which require a relatively low viscosity of below 20 Poise for application by the usual methods of spray, roller or brush, and particularly not for coating compositions which have to be applied and cure rapidly at ambient temperature, for example coatings for large structures such as ships, bridges, buildings, industrial plants, and oil rigs.
Coating compositions generally need to contain a polymer to confer film-forming properties, but any polymer used needs to be of sufficient molecular weight to give the required low viscosity, particularly after pigmentation as a paint. Such low-viscosity polymers often require long curing times to develop satisfactory mechanical properties, especially when cured at low temperature.
In WO 98/04594 a process is disclosed for the preparation of a curable polymer composition by polymerisation of a functional olefinically unsaturated monomer in the presence of a reactive diluent which is a liquid organic compound of viscosity less than 2 Pa·s (20 Poise) having at least one functional group which is substantially non-reactive with the functional olefinically unsaturated monomer and which is capable of reacting with a curing agent to form a polymer network. The major drawback of this technology is that this low-viscous coating material results in a low final film Tg and moderate durability.
In WO 96/16109 and WO 98/32792 an epoxy-polysiloxane coating composition is disclosed that is prepared by combining water, a polysiloxane, a difunctional aminosilane hardener, optionally an organooxysilane, and a non-aromatic epoxy resin. The maximum amount of solvent added to these compositions is approximately 420 g/l. The compositions are intended to be used as protective coatings for primed or galvanised steel, aluminium, concrete, and other substrates at a dry film thickness in the range of 25 μm to about two millimetres. Whilst these compositions are employed as durable topcoats, their gloss and colour retention properties when exposed to natural or accelerated test conditions (UV-A, UV-B) are not as expected for polysiloxane based compositions. This strongly affects the appearance of a coated substrate.
U.S. Pat. No. 4,446,259 discloses a coating composition having a liquid carrier and a binder which is a blend of an olefinically unsaturated polymer containing glycidyl groups and a crosslinkable polysiloxane having attached to the silicone atoms of its backbone alkyl, phenyl, and hydroxyl groups. These compositions are used as ambient temperature curing protective coatings. The major drawback of these compositions is the presence of a relatively large amount of organic solvent in the composition.
EP 0 822 240 discloses a coating resin composition comprising a silica-dispersed oligomer solution of an organosilane, an acrylic resin, and a curing catalyst. The coating resin compositions on average have a solid content in the range of 40-50% by weight. Consequently, these coating compositions have a VOC well above 250 g/l.
The present invention provides a solution to the drawbacks associated with the above-mentioned prior art.